Method of controlling susceptance of a post type obstacle



July 14, 1953 R, B, READE 2,645,679

METHOD OF CONTROLLING SUSCEPTANCE OF A POST TYPE OBSTACLE Filed NOV. 29, 1947 Fmi l INVENTOR. EaP/ff 5. P5455 Patented July 14, 1953 METHOD OF CONTROLLING SUSCEPTANCE F A POST TYPE OBSTACLE Ralph B. Reade, Rye, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application November 29, 1947, Serial No. 788,855 3 claims. (c1. .ssa- 73) This invention relates to electric wave propagating systems employing elements such as wave guides, or resonant cavities. While the idea of providing susceptance control obsta-cles within a wave guide has heretofore been proposed, the control by such obstacles has, in the main, been limited by the internal dimensions of the guide. In accordance with the present invention, the susceptance or impedance adjustment is effected by a novel obstacle arrangement which permits a very wide adjustment of the wave guide susceptance or impedance, which adjustment is not limited by the physical internal dimensions of the guide.

A principal object of the invention relates to an improved method for controlling the wave impedance at any desired section within alhollow wave guide.

Another object is to provide an improved device for adjusting the susceptance within a hollow wave guide.

Another object is to provide an improved wave filter for use in micro-wave systems.

A feature of the invention relates to a susceptance control element for use within a hollow 4wave guide and being of the obsta-cle type having simplied means for adjusting the susceptance .characteristics of the wave guide.

Another feature relates to a susceptance control element for use within a hollow wave guide, the element being of the obstacle type and being in the form of a coaxial transmission line section.

Another feature relates to a susceptance controlling obstacle for use within a hollow wave guide, and consisting of relatively telescopi-callyspaced elements by means of which the obstacle can be adjusted to provide either a capacitive reactance or an inductive reactance.

Another feature relates to a susceptance control obstacle for use within a hollow wave guide which consists of relatively telescopically-adjustable members for maintaining constant susceptance 0r reactance between the two sections of the guide on opposite sides of the obstacle.

A further feature relates to the novel construction of an obstacle-type wave impedance control device for use within hollow wave guides whereby the problems of mechanical tolerances, assembly, and adjustment of the obstacle, are greatly reduced.

,trol the Wave impedance.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein,

Fig. 1 is a perspective view of a hollow wave guide employing the improved susceptanceconp trolling device according to the invention.

Fig. 2 is a sectional view of Fig. 1 taken along the line 2-2 thereof.

Fig. 3 is a sectional view of Fig. 2 taken along the line 3 3 thereof.

Fig. 4 represents the prior known type of susceptance control obstacle which is used in eX- plaining the action of the obstacle of Figs. 1, 2 and 3 representing the invention.

Figs. 4A and 4B are equivalent electrical analogues to the arrangement of Fig. 4.

Figs. 5 and 6 are graphs explanatory of Fig. 4j

Fig. 7- is an equivalent schematic diagram of one adjustment of the susceptance controlling device of Figs. 1 to 3.

Fig. 8 is an equivalent schematic diagram representing another adjustment of the susceptance control device of Figs. 1, 2 and 3.

Fig. 9 is a modication of` Figs. 1-3.

For purposes of explanation, the invention will be illustrated as applied to a wave guide su-ch as is used in micro-wave and ultra high frequency transmission system. It will be understood however that the invention is equally useful in connection with any resonant cavity device, and finds its particular utility in connection with micro-Wave lters.

In such devices as micro-wave lters, employing a resonant cavity, it is necessary to control, among other things, the amount of coupling at theinput end and at the output end of the cavity. It is this value of coupling susceptance which determines the Q band width of the lter. Likewise in the case of hollow wave guides, it may be necessary to control the wave impedance at any particular section of the wave guide. It is well-known that obstacles in the nature of solid posts, irises, or diaphragms can be inserted within a hollow wave guide or resonant cavity to con- However, since in most cases the coupling susceptance of such fixed obstacle devices having a iixed geometric shape,

varies considerably as a function of frequency, it is necessary that some control be provided so that the coupling susceptance can be adjusted expeditiously to the proper value to achieve the desired Q. Furthermore, these prior known susceptance control posts or obstacles are limited by the inter-wall spacing of the wave guide within which they Aare mounted. In the case of micro-wave iilters of the constant band width type, this problem of controlling the susceptance of obstances is of considerable importance. For example, it may be necessary in one case to provide an inductive susceptance, and in another case to provide a capacitive susceptance. When physical obstacles having a symmetrical geometrical coniiguration are used within the cavity or wave guide, they tend to give rise to undesirable wave modes of propagation, for example, the TEM mode which results in a strong intercoupling between adjacent cavities. This is an undesirable elect for a number of reasons. In accordance with the invention, the susceptance controlling device is in the nature of a physical obstacle which, however, is designed so as toA be completely symmetrical from a geometrical standpoint, and whose range of impedance control is not ixed by the inter-wall spacing of the guide. Furthermore, the device can be easily adjusted externally of the cavity or wave guide to provide a trimmer susceptance which is isolated from the wave guide in such a way that it cannot excite the TE2,0 mode of vibration.

Such a device is shown in Figs. 1, 2 and 3 of the drawing. In these figures, the numeral I represents a portion of a hollow -wave guide or a resonant cavity which is connected to a suitable source of ultra high frequency or micro-waves. Preferably the excitation is such that the wave propagation is effected by the dominant TE mode, for example the TEo,1 mode. Thus in the case of the rectangular wave guide shown in Fig. 1, the excitation is such that the E-vector lines extend transversely across the narrow dimension of the guide, as illustrated more particularly in Fig. 2. Attached conductively-to and passing through the lower wall 2; of the wave guide and centrally thereof, is a tubular metal or conductive obstacle 3 which is in the form of a hollow cylinder having a portion extending inwardly and outwardly of the guide. Passing through the topV wall 4 of the guide is a metal rod 5 which is so arranged that it is coaxial within the member 3 and is arranged to be telescopically-adjusted with respect to the member 3. For example, if desired, the rod 5 may be threaded through the wall 4 for adjustment purposes. It will be seen, therefore, that the members 3 and 5 constitute in eilect a coaxial line section with the axis of the line extending parallel to the E-vector lines of the waves being propagated through the wave guide.

For the purpose of explanation of the arrangement shown in Figs. 1, 2 and 3, it will be assumed first that the members 3 and 5 are replaced by a simple cylindrical rod or obstacle 6., as shown in Fig. 4. The equivalent electrical analogue of the arrangement of Fig. 4 is shown in either Fig. 4A or Fig. 4B. However with this arrangement,

the susceptance will vary with frequency as illustrated by the graph of Fig. 5, or, in other words, the reactance will vary with frequency as illustrated in the graph of Fig. 6. In order to maintain constant susceptance or reactance, and assuming the inductive susceptance at the lower frequency end to be correct, it is necessary to add in series with the inductive susceptance yXL, a capacitance. Under this condition, the susceptance at the higher frequency end is equal to the susceptance at the lower frequency end, and is equal to 1 JiXLi-Xc) This condition is represented by the equivalent electrical circuit of Fig. 7. Thus by appropriately trimming jXc, it is possible to maintain constant reactance or susceptance, as seen across 'element is consequently in series with the element 3.V The capacitance is actually derived from the input impedance of the open-circuited coaxial line formed by the trimining rod 5 and the inner wall of element 3.v As long as this coaxial line is less than one-quarter wavelength, the reactance of the line will be capacitive. When the rod 5 is completely withdrawn from element 3 so as tev provide no telescopic overlap therewith, the reactance will be maximum. On the other hand, `when the rod 5 telescopically overlaps the element 3 by one-quarter wavelength, the reactance is zero. By adjusting the rod 5l beyond the quarter-wavelength position, for example between k/l and M2, it is possible to impart to the obstacle a variable inductive reactance represented schematically in Fig. 8.

While the elements 3 and 5 are shown in Figs. 1 to 3 as constituting an open-circuited coaxial line, it will be understood of course that they may be used as a shorted coaxial line. Thus the lower end of element 5 may be provided with a metal disc 6 (Fig. 9), attached thereto, it being understood that the disc is in good electrical contact both with the element 3 and the element 5.

While the drawing shows the invention as embodied in a rectangular wave guide, it will be understood that it can equally well be applied to any other cross-sectional shape of wave guide, `whether circular, elliptical, or other cross-sectional configuration.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. A susceptance control arrangement for hollow wave guides having walls defining a longitudinal channel, comprising a tubular obstacle extending tranversely and symmetrically of said channel through one wall of theguide interiorly thereof, and a rod extending through the opposite wall of the guide and entering into telescopic arrangement with the said tubular obstacle for a part of the length of said tubular obstacle, said rod having screw threadsV threadably received in said opposite wall whereby turning movement of said rod effects adjustment of said rod relative to said waveguide.

2. A susceptance control arrangement for hollow wave guides having Vwalls deiining a channel to propagate electric waves bythe dominant TE mode, comprising a tubular obstacle 'disposed within the guide transversely of lsaidV channel and attached to one of the walls thereof, a rod extending through the opposite wall of the guide into spaced telescopic relation with said tubular member to dene therewith a length of coaxial transmission line with its propagational axis in line with theelectrical Vector of the waves being propagated through the Wave guide and means for adjusting the length of said coaxial transmission line, said rod having screw threads threadably received in said opposite Wall whereby turning movement of said rod effects adjustment of said rod relative to said wave guide.

3. A susceptance control arrangement according to claim 1 further including a conductive disc disposed at the end of said rod for sliding contact with the interior of said tubular obstacle.

. RALPH B. READE.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date Zottu Dec. 21, 1943 Beck Sept. 24, 1946 Crosby Mar. 4, 1947 Carter Mar. 18, 1947 Woodward June 10, 1947 Zaleski Dec. 23, 1947 Southworth July 11, 1950 Wheeler Aug. 22, 1950 

